Synergistically Coupling Black Phosphorus Quantum Dots with MnO2 Nanosheets for Efficient Electrochemical Nitrogen Reduction Under Ambient Conditions.

The electrochemical nitrogen reduction reaction (NRR) is a promising strategy of nitrogen fixation into ammonia under ambient conditions. However, the development of electrochemical NRR is highly bottlenecked by the expensive noble metal catalysts. As a representative 2D nonmetallic material, black phosphorus (BP) has the valence electron structure similar to nitrogen, which can effectively adsorb the inactive nitrogen molecule and activate its triple bond. In addition, the relatively weak hydrogen adsorption can restrict the competitive and vigorous hydrogen evolution reaction. Herein, ultrafine BP quantum dots (QDs) are prepared via liquid-phase exfoliation and then assembled on catalytically active MnO2 nanosheets through van der Waals interactions. The obtained BP QDs/MnO2 catalyst demonstrates admirable synergetic effects in electrochemical NRR. The monodisperse BP QDs providing major activity manifest excellent ammonia production steadily with high selectivity, which benefits from the robust confinement of the BP QDs on the wrinkled MnO2 nanosheets with decent activity. A high ammonia yield rate of 25.3 µg h-1 mgcat. -1 and faradic efficiency of 6.7% can be achieved at -0.5 V (vs RHE) in 0.1 m Na2 SO4 electrolyte, which are dramatically superior to either component. The isotopic labelling and other control tests further exclude the external contamination possibility and attest the genuine activity.

Superconductivity in Pristine 2H_{a}-MoS_{2} at Ultrahigh Pressure.

As a follow-up of our previous work on pressure-induced metallization of the 2H_{c}-MoS_{2} [Chi et al., Phys. Rev. Lett. 113, 036802 (2014)PRLTAO0031-900710.1103/PhysRevLett.113.036802], here we extend pressure beyond the megabar range to seek after superconductivity via electrical transport measurements. We found that superconductivity emerges in the 2H_{a}-MoS_{2} with an onset critical temperature T_{c} of ca. 3 K at ca. 90 GPa. Upon further increasing the pressure, T_{c} is rapidly enhanced beyond 10 K and stabilized at ca. 12 K over a wide pressure range up to 220 GPa. Synchrotron x-ray diffraction measurements evidenced no further structural phase transition, decomposition, and amorphization up to 155 GPa, implying an intrinsic superconductivity in the 2H_{a}-MoS_{2}. DFT calculations suggest that the emergence of pressure-induced superconductivity is intimately linked to the emergence of a new flat Fermi pocket in the electronic structure. Our finding represents an alternative strategy for achieving superconductivity in 2H-MoS_{2} in addition to chemical intercalation and electrostatic gating.

Structural evolution behavior of manganese monophosphide under high pressure: experimental and theoretical study.

The influence of external pressure on the structural properties of manganese monophosphides (MnP) at room temperature has been studied using in situ angle dispersive synchrotron x-ray powder diffraction (AD-XRD) with a diamond anvil cell. The crystal structure of MnP is stable between 0 to 15 GPa. However, the compressibility of b-axis is much larger than those of a- and c-axes. From this result we suggested that the occurrence of superconductivity in MnP was induced by suppression of the long-range antiferromagnetically ordered state rather than a structural phase transition. Furthermore, the present experimental results show that the Pnma phase of MnP undergoes a pressure-induced structural phase transition at ~15.0 GPa. This finding lighted up-to-date understanding of the common prototype B31 structure (Strukturbericht Designation: B31) in transition metal monophosphides. No additional structural phase transition was observed up to 35.1 GPa (Run 1) and 40.2 GPa (Run 2) from the present AD-XRD results. With an extensive crystal structure searching and ab initio calculations, we predict that MnP underwent two pressure-induced structural phase transitions of Pnma → P213 and P213 → Pm-3m (CsCl-type) at 55.0 and 92.0 GPa, respectively. The structural stability and the electronic structures of manganese monophosphides under high pressure are also briefly discussed.

Structural phase transitions of (Bi1-xSbx)2(Te1-ySey)3 compounds under high pressure and the influence of the atomic radius on the compression processes of tetradymites.

Recently, A2B3-type tetradymites have developed into a hot topic in physical and material research fields, where the A and B atoms represent V and VI group elements, respectively. In this study, in situ angle-dispersive X-ray diffraction measurements were performed on Bi2Te2Se, BiSbTeSe2, and Sb2Te2Se tetradymites under high pressure. Bi2Te2Se transforms from a layered rhombohedral structure (phase I) into 7-fold monoclinic (phase II) and body-centered tetragonal (phase IV) structures at about 8.0 and 14.3 GPa, respectively, without an 8-fold monoclinic structure (phase III) similar to that in Bi2Te3. Thus, the compression behavior of Bi2Te2Se is the same as that of Bi2Se3, which could also be obtained from first-principles calculations and in situ high-pressure electrical resistance measurements. Under high pressure, BiSbTeSe2 and Sb2Te2Se undergo similar structural phase transitions to Bi2Te2Se, which indicates that the compression process of tellurides can be modulated by doping Se in Te sites. According to these high-pressure investigations of A2B3-type tetradymites, the decrease of the B-site atomic radius shrinks the stable pressure range of phase III and expands that of phase II, whereas the decrease of the A-site atomic radius induces a different effect, i.e. expanding the stable pressure range of phase III and shrinking that of phase II. The influence of the atomic radius on the compression process of tetradymites is closely related to the chemical composition and the atom arrangement in the quintuple layer.

Structural Phase Transitions and Metallized Phenomena in Arsenic Telluride under High Pressure.

In this study, first-principle calculations, in situ angle-dispersive X-ray diffraction, and in situ electrical resistance measurements were performed on arsenic telluride (As2Te3) under high pressure. Structural phase transitions and metallized phenomena were observed from the calculated and experimental results. Upon compression, α-As2Te3 transforms into phases α' and α″ at ∼5.09 and ∼13.2 GPa, respectively, with two isostructural phase transitions. From 13.2 GPa, As2Te3 starts to transform into phase γ, with one first-order monoclinic to monoclinic crystal structural phase transition. According to the first-principle calculations and electrical resistance measurements, the structural phase transitions in the compression process induce the transformation from an insulator (phase α) across a semimetal (phase α') into a metal (phases α″ and γ). The evolution of the structure and transport property upon compression on As2Te3 is helpful for understanding the properties of other A2B3-type compounds under high pressure.

Anomalous anisotropic compression behavior of superconducting CrAs under high pressure.

CrAs was observed to possess the bulk superconductivity under high-pressure conditions. To understand the superconducting mechanism and explore the correlation between the structure and superconductivity, the high-pressure structural evolution of CrAs was investigated using the angle-dispersive X-ray diffraction (XRD) method. The structure of CrAs remains stable up to 1.8 GPa, whereas the lattice parameters exhibit anomalous compression behaviors. With increasing pressure, the lattice parameters a and c both demonstrate a nonmonotonic change, and the lattice parameter b undergoes a rapid contraction at ∼ 0.18-0.35 GPa, which suggests that a pressure-induced isostructural phase transition occurs in CrAs. Above the phase transition pressure, the axial compressibilities of CrAs present remarkable anisotropy. A schematic band model was used to address the anomalous compression behavior of CrAs. The present results shed light on the structural and related electronic responses to high pressure, which play a key role toward understanding the superconductivity of CrAs.

Structural phase transitions in Bi2Se3 under high pressure.

Raman spectroscopy and angle dispersive X-ray diffraction (XRD) experiments of bismuth selenide (Bi2Se3) have been carried out to pressures of 35.6 and 81.2 GPa, respectively, to explore its pressure-induced phase transformation. The experiments indicate that a progressive structural evolution occurs from an ambient rhombohedra phase (Space group (SG): R-3m) to monoclinic phase (SG: C2/m) and eventually to a high pressure body-centered tetragonal phase (SG: I4/mmm). Evidenced by our XRD data up to 81.2 GPa, the Bi2Se3 crystallizes into body-centered tetragonal structures rather than the recently reported disordered body-centered cubic (BCC) phase. Furthermore, first principles theoretical calculations favor the viewpoint that the I4/mmm phase Bi2Se3 can be stabilized under high pressure (>30 GPa). Remarkably, the Raman spectra of Bi2Se3 from this work (two independent runs) are still Raman active up to ~35 GPa. It is worthy to note that the disordered BCC phase at 27.8 GPa is not observed here. The remarkable difference in atomic radii of Bi and Se in Bi2Se3 may explain why Bi2Se3 shows different structural behavior than isocompounds Bi2Te3 and Sb2Te3.

Conventional empirical law reverses in the phase transitions of 122-type iron-based superconductors.

Phase transition of solid-state materials is a fundamental research topic in condensed matter physics, materials science and geophysics. It has been well accepted and widely proven that isostructural compounds containing different cations undergo same pressure-induced phase transitions but at progressively lower pressures as the cation radii increases. However, we discovered that this conventional law reverses in the structural transitions in 122-type iron-based superconductors. In this report, a combined low temperature and high pressure X-ray diffraction (XRD) measurement has identified the phase transition curves among the tetragonal (T), orthorhombic (O) and the collapsed-tetragonal (cT) phases in the structural phase diagram of the iron-based superconductor AFe2As2 (A = Ca, Sr, Eu, and Ba). The cation radii dependence of the phase transition pressure (T → cT) shows an opposite trend in which the compounds with larger ambient radii cations have a higher transition pressure.

Pressure-induced phase transitions and correlation between structure and superconductivity in iron-based superconductor Ce(O(0.84)F(0.16))FeAs.

High-pressure angle-dispersive X-ray diffraction experiments on iron-based superconductor Ce(O(0.84)F(0.16))FeAs were performed up to 54.9 GPa at room temperature. A tetragonal to tetragonal isostructural phase transition starts at about 13.9 GPa, and a new high-pressure phase has been found above 33.8 GPa. At pressures above 19.9 GPa, Ce(O(0.84)F(0.16))FeAs completely transforms to a high-pressure tetragonal phase, which remains in the same tetragonal structure with a larger a-axis and smaller c-axis than those of the low-pressure tetragonal phase. The structure analysis shows a discontinuity in the pressure dependences of the Fe-As and Ce-(O, F) bond distances, as well as the As-Fe-As and Ce-(O, F)-Ce bond angles in the transition region, which correlates with the change in T(c) of this compound upon compression. The isostructural phase transition in Ce(O(0.84)F(0.16))FeAs leads to a drastic drop in the superconducting transition temperature T(c) and restricts the superconductivity at low temperature. For the 1111-type iron-based superconductors, the structure evolution and following superconductivity changes under compression are related to the radius of lanthanide cations in the charge reservoir layer.

High-pressure phase transitions, amorphization, and crystallization behaviors in Bi2Se3.

The phase transition, amorphization, and crystallization behaviors of the topological insulator bismuth selenide (Bi2Se3) were discovered by performing in situ high-pressure angle-dispersive x-ray diffraction experiments during an increasing, decreasing, and recycling pressure process. In the compression process, Bi2Se3 transforms from the original rhombohedral structure (phase I(A)) to a monoclinic structure (phase II) at about 10.4 GPa, and further to a body-centered tetragonal structure (phase III) at about 24.5 GPa. When releasing pressure to ambient conditions after the complete transformation from phase II to III, Bi2Se3 becomes an amorphous solid (AM). In the relaxation process from this amorphous state, Bi2Se3 starts crystallizing into an orthorhombic structure (phase I(B)) about five hours after releasing the pressure to ambient. A review of the pressure-induced phase transition behaviors of A2B3-type materials composed from the V and VI group elements is presented.

Pressure-induced disordered substitution alloy in Sb2Te3.

A new type of disordered substitution alloy of Sb and Te at above 15.1 GPa was discovered by performing in situ high-pressure angle-dispersive X-ray diffraction experiments on antimony telluride (Sb(2)Te(3)), a topological insulator and thermoelectric material, at room temperature. In this disordered substitution alloy, Sb(2)Te(3) crystallizes into a monoclinic structure with the space group C2/m, which is different from the corresponding high-pressure phase of the similar isostructural compound Bi(2)Te(3). Above 19.8 GPa, Sb(2)Te(3) adopts a body-centered-cubic structure with the disordered atomic array in the crystal lattice. The in situ high-pressure experiments down to about 13 K show that Sb(2)Te(3) undergoes the same phase-transition sequence with increasing pressure at low temperature, with almost the same phase-transition pressures.

Structural and physical properties of the 6M BaIrO(3): a new metallic iridate synthesized under high pressure.

The 6M BaIrO(3) with the distorted hexagonal BaTiO(3) structure was synthesized by high-pressure sintering. Through Rietveld refinement of the powder X-ray diffraction data, the lattice parameters of a = 5.7459(1) A, b = 9.9289(2) A, c = 14.3433(2) A, and beta = 91.340(1) degrees were obtained. In the Ir(2)O(9) dioctahedron, the average Ir-O distance and direct Ir-Ir distance were equal to 2.067(19) and 2.719(1) A, respectively. The temperature dependence of electrical resistivity shows that the 6M BaIrO(3) is a new metallic iridate. It is an abnormal metal, being deviated from the Fermi liquid behavior, following a linear relationship of rho versus T below 20 K. Both magnetic susceptibility and specific heat data indicate that it is an exchange-enhanced Pauli paramagnet, because of the electron-electron correlation effect.

Struture stability and compressibility of iron-based superconductor Nd(O0.88F0.12)FeAs under high pressure.

The high-pressure angle-dispersive X-ray diffraction experiments on the iron-based superconductor Nd(O0.88F0.12)FeAs were performed up to 32.7 GPa at room temperature. An isostructural phase transition starts at approximately 10 GPa. When pressure is higher than 13.5 GPa, Nd(O0.88F0.12)FeAs completely transforms to a high-pressure phase, which remains the same tetragonal structure with a larger a-axis and smaller c-axis than those of the low-pressure phase. The ambient conditions isothermal bulk moduli B0 are derived as 102(2) and 245(9) GPa for the low-pressure phase and high-pressure phase, respectively. The structure analysis based on the Rietveld refinement methods shows the difference of pressure dependence of the Fe-As and Nd-(O, F) bonding distances, as well as As-Fe-As and Nd-(O, F)-Nd angles between the low-pressure phase and high-pressure phase.